1. Field of the Invention
The invention relates to core barrel apparatus for obtaining and retrieving sealed samples of subterranean formations and, more particularly, to expansion chambers and valves thereof.
2. Description of the Prior Art
In the art of obtaining samples of subterranean formations it is well known to lower a coring device or other formation sampling apparatus into a well bore while suspended by a cable, a wire line, or a drill string. It is desired that not only the solid core sample but also any fluids present therein be recovered and brought to the surface. Retaining fluids in the sampling apparatus, particularly the compressible gaseous fluids, is difficult and unreliable due to the extreme pressure differential between the sealed interior of the apparatus, in which is present the greatly elevated formation pressure at the bottom of the bore hole, and the ambient atmospheric pressure when the sampling apparatus is brought to the surface. This pressure differential varies as a function of the depth of the bore hole; however it is not uncommonly in the range of about 5,000-11,000 pounds per square inch.
Some known core barrel devices and sampling methods seal the sample at the formation pressure in a fluid-tight chamber which is capable of withstanding a high internal pressure while maintaining a fluid-tight seal as the sample is brought to the surface. Such fluid-tight chambers are provided with extremely strong and reliable seals, walls, etc. and yet are still prone to leak. Use of such chambers presents a safety hazard in that the high pressure within the chamber makes the sampling device difficult and perhaps even dangerous to handle, particularly when moving the chamber or attempting to withdraw the fluid and solid samples from the chamber.
It has been disclosed in U.S. Pat. No. 2,287,909, issued on June 30, 1942 to B. W. Sewell, that the pressure differential may be reduced by providing an expandable volume within the sampling apparatus into which volume the gases may expand to reduce the pressure differential. The sampling apparatus of the Sewell patent may not be adaptable for conditions encountered in comparatively deep bore holes due to the size of the expansion chamber shown in the Sewell '909 patent. By way of example, the bottom hole conditions in a bore hole of approximately 10,000 feet in depth could be from about 5,000-11,000 psig and 200.degree. F. Gas or vapor under such bottom hole conditions is of a density that will cause it to expand from approximately 200-400 times in volume when elevated to surface conditions. Further by way of example, if the core sample to be taken is approximately 4 inches in diameter and 20 feet in length and if the rock sample is 20% porous with half of the porous region occupied by gas, the gas could expand to approximately 70 cubic feet when elevated to surface conditions. Accordingly, a sampling apparatus for a deep bore hole must be provided with an expansion chamber capable of expanding to an appreciable volume. Furthermore, the apparatus disclosed in the Sewell '909 patent does not provide a selectively closed expansion chamber which may be sealed from drilling fluid and other substances as the device is lowered in the well bore and as a sample is taken. Thus, the chamber of the Sewell '909 patent may become filled with drilling fluid or other unwanted substances as the core sample is being obtained. Receiving drilling fluid or other substances in the expansion chamber of the device of the Sewell '909 patent not only can detract from the usefulness of the formation sample but also volume in the chamber is occupied which could otherwise provide additional volume for gaseous components of the sample as they expand. Additionally, the chamber in which the sample is received and the expansion chamber of the Sewell '909 patent may not be individually sealed and separated.
It is apparently unknown in the prior art to maintain a sample chamber sealed or closed until opened at the bottom of a bore hole and to expand the sample chamber in response to the gases therein as the sample is brought to the surface.
The present invention overcomes the aforementioned drawbacks and disadvantages of the prior art and provides additional advantages as well.